How does the GPL apply to code in Blender files? (e.g. Drivers)
Short answer: Yes, the GPL applies to drivers too.

Just as the GPL applies to addons, it applies to drivers (any scripts running inside of Blender). However this doesn't automatically everything you do in that blend file is suddenly GPL.

Consider:


In many cases drivers on their own don't constitute an original work, Obviously you can't copyright 1 + 1, rot / pi or other simple expressions.Exactly what is original enough to be copyrighted isn't totally clear and depends on local laws.
In the case where the client/customer has the blend file, they can see the driver expressions anyway, and use them, however drivers are rarely useful on their own, typically they are only valuable because of their use within a rig and data they reference (the rig, objects etc).
In many cases, the deliverable content from a project isn't a blend file, The finished work may be image / video / exported-model, therefor there is no need to distribute the blend file, a driver can be treated as in house software.




So the main case this prohibits are:


If you made a rig and wanted to charge licensing fee's for using the drivers.While this remains possible, you would have to use a GPL compatible license for the drivers.
If you give a blend file to someone, you could not prohibit them from reusing or sharing a driver-expression in that blend file.


In practice, the drivers in a blend file are plain-text anyway, and not all that useful on their own.



But might mixing the scripts with the .blend file data, also make the blend file GPL?

It's worth looking into this more, however Wordpress lets images in a theme be exempt from the GPL. (Where the theme may contain code &amp; data, this is a precedent where the data is exempt while the code isn't).
http://wordpress.org/news/2009/07/themes-are-gpl-too/



Obligatory - I am not a lawyer disclaimer, however this is how the GPL applies as I understand it @ideasman42